1. Field of the Invention
The present invention relates to a toner-density calculating method, a reflective optical sensor that is used in the toner-density calculating method, and an image forming apparatus. The image forming apparatus is provided, for example, in a multifunction product (MFP) that includes the reflective optical sensor and works as at least one of a copier, a printer, a facsimile machine, and a plotter.
2. Description of the Related Art
A variety of image forming apparatuses use toner to form images, i.e., they form toner images. Example of such image forming apparatuses are analog image forming apparatuses, digital image forming apparatuses, black-and-white copiers, color copiers, printers, plotters, facsimile machines, and, multifunction printers (MFPs).
To form a good quality toner image, as is widely known, an electrostatic latent image needs to be developed with just an appropriate amount of toner. The electrostatic latent image can be developed with a two-component developer that contains toner and carrier or a single-component developer that contains only toner. An amount of the toner to be supplied to a developing unit that develops the electrostatic latent image is called, hereinafter, “toner density”.
If the toner density is low, i.e., if the amount of the toner supplied to the electrostatic latent image is less than the necessary amount, a paler toner image will be formed. If the toner density is high, i.e., if the amount of the toner supplied to the electrostatic latent image is more than the necessary amount, a darker and difficult-to-see toner image will be formed. To form a good quality toner image, the toner density should be within an appropriate range.
To adjust the toner density to a value within the appropriate range, it is necessary to calculate the current toner density. In a typical method, the toner density is calculated from a change in a detection light reflected by a toner image that is formed dedicated to the toner-density calculation (hereinafter, “toner pattern”). An optical device that emits the detection light to the toner pattern and receives the detection light reflected by the toner pattern is called a reflective optical sensor.
Various types of reflective optical sensors are known in the art (see Japanese Patent Application Laid-open No. 2008-064953, Japanese Patent Application Laid-open No. S64-35466, Japanese Patent Application Laid-open No. 2004-21164, Japanese Patent Application Laid-open No. 2002-72612, Japanese Patent Application Laid-open No. 2004-309292, and Japanese Patent Application Laid-open No. 2004-309293).
Typical reflective optical sensors include a light-emitting unit and a light-receiving unit. The light emitting unit includes one, two, or three light-emitting elements having different wavelength characteristics. The light-receiving unit includes one or two light-receiving elements (e.g., photodiodes (PDs) or phototransistors).
Light-emitting diodes (LEDs) are typically used as the light-emitting elements. The LEDs emit the detection light of a spot size that is smaller than the toner pattern on the toner pattern.
The toner pattern is formed, for example, on a transfer belt. The toner pattern moves as the transfer belt rotates. A direction in which the transfer belt moves due to the rotation is called “sub-direction”, and a direction perpendicular to the sub-direction on the transfer belt is called “main-direction”. In a system in which electrostatic latent images are formed through optical scanning, the main-direction corresponds to the main-scanning direction, and the sub-direction corresponds to the sub-scanning direction.
An electrostatic latent image corresponding to a toner pattern is formed on a photosensitive member by optically scanning a surface of the photosensitive member with an electrostatic-latent-image forming unit, and the electrostatic latent image on the surface of the photosensitive member is then developed into the toner pattern. The toner pattern on the photosensitive member is then transferred onto the transfer belt, and is moved in the sub-direction with the rotation of the transfer belt. When the toner pattern enters a detection area, the toner pattern is exposed with a spot of the detection light from the reflective optical sensor.
The spot size of the spot of the detection light is typically about 2 millimeters (mm) to 3 mm.
In an ideal situation, the spot falls on the center of the toner pattern in the main-direction. However, it is difficult to always keep a relative position between the toner pattern and the reflective optical sensor in the main-direction the ideal state, due to various reasons. These reasons include fluctuation in an optical scanning area of the electrostatic-latent-image forming unit, the transfer belt meandering, which is positional shift of the reflective optical sensor in the main-direction from an initial installation position because of degradation while time passes.
If a portion of the spot falls in a region where there is no toner pattern because of the positional mismatch in the main-direction between the toner pattern and the reflective optical sensor, the reflected light received by the light-receiving unit represents wrong data, and therefore the calculated toner density is wrong. Assume, for example, that one light-emitting element emits one spot of the detection light, one light-receiving element receives the reflected light, and the toner density is calculated from a difference between specularly reflected light and diffusely reflected light. The light-receiving element is arranged to receive the specularly reflected light. If a first portion of the spot falls in a region where there is no toner pattern and a second portion falls on the toner pattern, the first portion of the detection light is reflected specularly while the second portion is reflected diffusely. As a result, in a configuration where the light-receiving element is arranged so as to receive the specularly reflected light, as compared to a case where the entire spot falls out of the toner pattern, intensity of the specularly reflected light that is received at the light-receiving element decreases due to the generation of the diffusely reflected light. The decrease in the intensity of the specularly reflected light can also occur when the toner amount at the toner pattern is low. Therefore, whether the decrease in the intensity of the specularly reflected light is due to low toner amount or mismatch between the spot and the toner pattern is always unclear.
To solve this problem, in the conventional techniques, the toner pattern of a size from about 15 mm to about 25 mm in both the main-direction and the sub-direction is formed so that the spot of the detection light cannot fall out of the toner pattern even in case of the positional mismatch.
In the image forming apparatuses, specifically, in the color image forming apparatus, the detection of the toner density by the reflective optical sensor using the toner pattern is performed to acquire so as to maintain high image quality as a maintenance activity necessary for an accurate image-forming process. Because the toner-density calculation is performed as the maintenance activity separated from the main activity, i.e., an image-forming process, the image forming operation cannot be performed during the toner-density calculation.
When the electrostatic latent image to be developed as the toner pattern is written by the optical scanning, a time period required for the optical scanning becomes longer as the size of the toner pattern become larger. In other words, the larger the toner pattern is, the lower the operating efficiency of the image formation becomes.
Moreover, because a total amount of the toner in the toner container or the like is fixed, as an amount of the toner to be used for the toner pattern increases, an amount of the toner to be used for the main activity also increases, i.e., the image formation decreases disadvantageously. The larger the toner pattern is, the more the toner amount is consumed for the toner pattern. In this manner, the conventional toner-density measuring methods have the two disadvantages, i.e., the low operating efficiency and the much toner-consumption amount for the toner pattern.
An array-type reflective optical sensor that solves the above problems is disclosed in Japanese Patent Application Laid-open No. 2008-070198 by the applicant of the present application.
The reflective optical sensor includes a light-emitting unit and a light-receiving unit. The light-emitting unit includes M number of light-emitting elements (M≧3) that each emits detection light to a supporting member. The light-emitting elements are aligned in a single direction intersecting with the sub-direction in such a manner that M number of spots of detection light fall on the supporting member and the distance between adjacent spots in the direction perpendicular to the sub-direction is less than or equal to the length of the toner pattern. The light-receiving unit includes N number of (N≧3) light-receiving elements. The light-receiving elements are aligned in a single direction on a plane opposed to the supporting member at positions corresponding to the light-emitting unit so as to receive the detection light reflected by the supporting member and/or the toner pattern.
The toner density is calculated on the basis of outputs from N number of the light-receiving elements that receive light emitted from the M number of light-emitting elements.
The supporting member is, typically, a transfer belt with a specular surface; therefore, light reflected from the transfer belt is specularly reflected light. Light reflected from the toner pattern includes both specularly reflected light and diffusely reflected light.
In Japanese Patent Application Laid-open No. 2008-070198, an embodiment is disclosed in which, when an arbitrary light-emitting element turns ON, the detection light reflected by the transfer belt (specularly reflected light) is received only at the light-receiving element corresponding to the on-state light-emitting element.
This means that, when an arbitrary light-emitting element turns ON and the detection light strikes the toner pattern, although the detection light is specularly and diffusely reflected by the toner pattern, specularly reflected light is received at only the light-receiving element corresponding to the on-state light-emitting element. M−1 number of the other light-receiving elements receive the diffusely reflected light.
Therefore, if the toner density is calculated with the above-described configuration on the basis of the amount of specularly reflected light, only those outputs are needed that are from the light-receiving element corresponding to an arbitrary on-state light-emitting element observed when light is reflected by the transfer belt and when light is reflected by the toner pattern. If the toner density is calculated on the basis of the amount of diffusely reflected light, in contrast, only those outputs are needed that are from the M−1 number of the light-receiving elements other than the light-receiving element corresponding to the on-state light-emitting element observed when light is reflected by the toner pattern.
However, the detection light reflected by the transfer belt (specularly reflected light) can be received not only by the light-receiving element corresponding to the on-state light-emitting element but also by some light-receiving elements near the corresponding light-receiving element (light-receiving elements not corresponding to the on-state light-emitting element).
Moreover, part of detection light that is specularly reflected by the toner pattern can be received not only by the light-receiving element corresponding to the on-state light-emitting element but also by some light-receiving elements not corresponding to the on-state light-emitting element.
If, in the above situation, only the output from the light-receiving element corresponding to the on-state light-emitting element is determined to be the output representing the light specularly reflected by the transfer belt and/or the toner pattern and the outputs from the light-receiving elements not corresponding to the on-state light-emitting element that are observed when the toner pattern is exposed to the light are determined to be the output representing the light diffusely reflected by the toner pattern, some of the outputs from the other light-receiving elements not corresponding to the on-state light-emitting element are partially not used in the toner-density calculation, which will lead to an inaccurate calculation result.
In other words, values of outputs (detection information) helpful to increase the sensitivity of the reflective optical sensor and the accuracy in the toner-density calculation are not properly used in the toner-density detection, resulting discarded uselessly.